Phenylalanine ammonia-lyase (PAL) is a non-mammalian enzyme widely distributed in plants (Koukol, et al., J. Biol. Chem. 236:2692-2698 (1961); Hanson, et al., The Enzymes 7:75-166 (1972); Poppe, et al., Curr. Org. Chem. 7:1297-1315 (2003)), some fungi (Rao, et al., Can. J. Biochem. 4512:1863-1872 (1967); Abell, et al., Methods Enzymol. 142:242-253 (1987)) and bacteria (Bezanson, et al., Can. J. Microbiol. 16:147-151 (1970); Xiang, et al., J. Biol. Chem. 277:32505-32509 (2002); Hill, et al., Chem. Commun. 1358-1359 (2003)) and can be recombinantly produced in Escherichia coli. 
PAL from two cyanobacteria strains, Anabaena variabilis (Av) and Nostoc punctiforme (Np), has been cloned and expressed in bacteria (e.g., Escherichia coli (E. coli), and was shown to display PAL enzyme activity in vitro and in vivo (see, e.g., U.S. Pat. Nos. 7,531,341; 7,534,595; 7,537,923; and 7,560,263). A pegylated recombinant Anabaena variabilis PAL (rAvPAL-PEG) has also been produced, wherein the rAvPAL protein was derivatized by covalent attachment of polyethylene glycol (PEG) to increase its half-life and optimize its pharmacokinetic profile and/or reduce its immunogenicity (Id.). rAvPAL-PEG has been shown to metabolize phenylalanine and is being developed as an enzyme substitution therapy (EST) for patients disorders or diseases associated with elevated levels of phenylalanine, such as HPA, including PKU, as well as in cancer therapy (Id.).
Although PAL potentially has various therapeutic applications, the use of PAL can be limited by reduced specific activity and proteolytic instability. Similar to other therapeutic proteins, use of PAL as an enzyme therapy is accompanied by several disadvantages such as immunogenicity and proteolytic sensitivity (see Vellard, Curr. Opin. Biotechnol. 14:1-7 (2003)). Further, a delicate balance is required between substrate affinity and enzyme activity to achieve and maintain control of plasma phenylalanine levels within a normal somewhat narrow range in disorders characterized by hyperphenylalaninemia. As yet, a concerted effort toward improving these parameters has not been made due to a paucity of structural and biochemical knowledge regarding this protein.
Thus, there remains a need for PAL molecules with optimal kinetic characteristics, including potent catalytic activity, greater biological half-life, greater biochemical stability, and/or attenuated immunogenicity, for therapeutic use, including the treatment of HPA, including PKU, and other disorders, including cancer.